Apparatus for enhancing condensation and boiling of a fluid

ABSTRACT

An apparatus enhances the condensation and boiling of a fluid in heat exchange machines, by directly applying time-periodic acoustic waves with a resonance oscillation frequency to liquid drops and/or bubbles formed on a solid surface, when the fluid is in the process of condensation or boiling, thereby effectively removing them therefrom. The apparatus comprises a signal generator for generating a driving signal based on a resonant oscillation frequency of at least one of the liquid drops and the bubbles; and a vibrator, in response to the driving signal, for providing an acoustic pressure wave to said at least one of the liquid drops and the bubbles, to thereby detach them from the solid surface.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a condensation and boilingsystem; and, more particularly, to an apparatus for enhancingcondensation and boiling of a fluid in heat exchange machines, byemploying the frequency characteristics of liquid drops and/or bubblesformed on a solid surface.

[0003] 2. Description of the Related Art

[0004] There are various techniques for promoting condensation andboiling of a fluid, performed in heat exchange machines such asrefrigerators, air-conditioners, and heaters. The techniques include:mechanically modifying a solid surface of, e.g., a tube or wall of theheat exchange machines, which is in contact with the flowing fluid inthe process of condensation or boiling; directly vibrating a solidsurface, such as a metal or mirror, with a resonant oscillationfrequency of the solid part not of the fluid, as disclosed in U.S. Pat.No. 5,025,187; applying an electric field to a fluid; and coating thesolid surface with a surfactant.

[0005] The surface modification technique mechanically creates aplurality of grooves on the solid surface to increase the total surfacearea thereof with which the fluid contacts. This technique, however, isnot gaining popularity since its processing cost is considerably high.In addition, the pressure drop of the flow is increased because thegrooves provided on the solid surface promotes the fouling and scalingin the condensation and boiling system during operation. Theelectric-field-applying technique has a problem of its own that asignificantly high voltage of several tens of kilovolts (kV) is requiredfor the generation of the electric field, thereby deteriorating thesafety thereof.

[0006] In the surfactant-coating technique, the performance ofcondensation and boiling of the fluid becomes lower as the coatedsurfactant is gradually dissolved, after a certain time, causing seriousenvironmental pollution due to the dissolved surfactant. Thesolid-vibration technique is difficult to apply for a shell-and-tubetype heat exchange machine and also requires vibration of several tensof kilohertz to be imposed on the system, which consumes a significantamount of energy.

SUMMARY OF THE INVENTION

[0007] Therefore, the objective of the present invention is to providean apparatus, which can be applicable to any condensation and boilingsystem employing various-shape heat exchange machines, for enhancingcondensation and boiling of a fluid in a simple and a verycost-effective manner. In the apparatus of the present invention,time-periodic acoustic pressure waves are directly applied to liquiddrops and/or bubbles formed on the solid surface, thereby effectivelydetaching the liquid drops and/or bubbles from the solid surface.Consequently, the condensation and boiling of the fluid, performed inthe system, is enhanced.

[0008] In accordance with the present invention, there is provided anapparatus for enhancing condensation and boiling of a fluid in heatexchange machines by separating at least one of liquid drops and bubblesfrom a solid surface with which the fluid contacts, the apparatuscomprising: a signal generator for generating a driving signal based ona resonant oscillation frequency of said at least one of the liquiddrops and the bubbles; and a vibrator, in response to the drivingsignal, for providing an acoustic pressure wave to said at least one ofthe liquid drops and the bubbles, to thereby detach them from said solidsurface.

BRIEF DESCRIPTIONS OF THE DRAWINGS

[0009] The above and other objectives and features of the presentinvention will become apparent from the following description of apreferred embodiment given in conjunction with the accompanyingdrawings, in which:

[0010]FIG. 1 shows a block diagram of an apparatus for enhancingcondensation and boiling of a fluid in accordance with the presentinvention;

[0011]FIG. 2 is a graphical representation illustrating the relationshipbetween a minimum vibration velocity that causes liquid drop detachmentand a vibration frequency imposed on the liquid drop; and

[0012]FIG. 3 presents photographs showing a series of removal processfor the liquid drop in accordance with the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0013] Referring now to FIG. 1, there is shown an apparatus 100 forenhancing condensation and boiling of a fluid, performed in heatexchange machines, in accordance with the present invention. As shown inFIG. 1, the apparatus 100 comprises a signal generator 102, an amplifier104, and a vibrator 106. Preferably, they may be designed as oneintegrated unit. The signal generator 102 produces an electrical signalwith a resonant oscillation frequency corresponding to the naturaloscillation frequency of liquid drops and/or bubbles 120. The electricalsignal may be a sinusoidal, saw-tooth, or rectangular wave. The liquiddrops and/or bubbles 120 are formed on a solid surface 110 when a fluidis in the process of condensation or boiling. The solid surface 110 iscontacting with the fluid during the condensation or boiling processthereof.

[0014] Thereafter, the electrical signal is provided to the amplifier104, which amplifies it to a predetermined signal level. The amplifiedsignal is provided to the vibrator 106 as a driving signal. In responseto the driving signal, the vibrator 106 generates time-periodic acousticpressure waves with the same frequency as that of the electrical signal.The time-periodic acoustic pressure waves are then applied to the liquiddrops and/or bubbles 120 formed on the solid surface 110. When thefrequency of the imposed vibration coincides with the resonantoscillation frequency of the liquid drops or bubbles 120, the liquiddrops or bubbles 120 oscillate very violently to eventually disengagefrom the solid surface 110 (hereinafter, the liquid drops and bubbles120 are referred to as liquid drops for the purpose of simplicity).Consequently, the condensation or boiling process of the fluid can beenhanced as a result of an increasing area where the surrounding fluidcould contact with the solid surface 110.

[0015] The vibrator 106 may be implemented by using an acoustic speaker,which is capable of easily generating the time-periodic acousticpressure waves. Alternatively, there may be employed a piston, cam,membrane, or flap associated with a motor, or a piezoelectric device,for the same purpose.

[0016] Referring to FIG. 2, there is a graphical representationillustrating the relationship between a minimum vibration velocity thatcauses drop detachment and the vibration frequency imposed on the liquiddrops 120 formed on the solid surface 110.

[0017] In FIG. 2, each point depicted as a circle represents a meanvalue of minimum vibration velocities of the vibrator 106, causing dropdetachment at each vibration frequency applied to the liquid drops 120as mentioned above. Each point is measured by increasing the vibrationamplitude from zero at a fixed vibration frequency. An error bar markedon each point represents the standard deviation of positive and negativeto the vibration velocity. The liquid drops 120 are composed of waterand each volume of those equals that of a sphere with the diameter of,approximately, 1.3 millimeter (mm).

[0018] As well known in the art, when the liquid drops 120 float in theair, the natural oscillation frequency ƒ thereof may be written asfollows: $\begin{matrix}{f = {\frac{1}{2\pi}\lbrack {{n( {n - 1} )}( {n + 2} )\frac{\sigma}{\rho \quad a^{3}}} \rbrack}^{\frac{1}{2}}} & {{Eq}.\quad (1)}\end{matrix}$

[0019] wherein n is a vibration mode number for determining the shape ofa vibration and σ, ρ, and a represent the surface tension, density, anddiameter of each of the liquid drops 120, respectively.

[0020] Similarly, when bubbles float in the air, the natural oscillationfrequency ƒ thereof may be written as follows: $\begin{matrix}{f = {\frac{1}{2\pi}\lbrack {( {n + 1} )( {n - 1} )( {n + 2} )\frac{\sigma}{\rho \quad a^{3}}} \rbrack}^{\frac{1}{2}}} & {{Eq}.\quad (2)}\end{matrix}$

[0021] wherein n is a vibration mode number for determining the shape ofa vibration and σ, ρ denote the surface tension and density of asurrounding liquid, respectively, and a is the diameter of each of thebubbles. (See, H. Lamb, Hydrodynamics, 6th Ed. Cambridge UniversityPress, Cambridge, England(1932), p.475.)

[0022] In case that the liquid drops 120 are waters, σ is to 0.0717 N/m,and ρ is to 1000 kg/m³. Calculating the natural oscillation frequency ƒof the liquid drops 120 by substituting the values of σ, ρ, and a intoEq. (1), the natural oscillation frequency ƒ becomes 80 Hz when thevibration mode number of the liquid drops 120 is two.

[0023] It is assumed that the resonant oscillation frequency of theliquid drops 120 contacting with the solid surface 110 is similar tothat of the liquid drops 120 floating in the air. Then, the resonantoscillation frequency of the liquid drops 120 formed on the solidsurface 110 will approximate to 80 Hz. This is consistent with ameasured point B in the proximity of 80 Hz, as shown in FIG. 2.Consequently, the measured point B proves to be the resonant oscillationfrequency of the liquid drops 120 when the vibration mode number thereofis two. And, a measured point A with a lower frequency than that of themeasured point B is supposed to be a resonant oscillation frequency whenthe vibration mode number thereof is one.

[0024] In addition, the fact that the minimum vibration frequency fordrop detachment is locally minimum at points A and B indicates that thevibrations whose frequencies correspond to the points A and Beffectively oscillate the liquid drops 120 to cause them to disengagefrom the solid surface 110. On the contrary, in the frequency rangesother than those near the points A and B, higher vibration velocity isrequired to induce drop detachment. Therefore, it is clear that imposingvibrations at resonant oscillation frequencies of the liquid drops 120are crucial in effectively promoting the drop disengagement process,thereby enhancing the condensation and boiling of the fluid.

[0025] Referring to FIG. 3, there is presented photographs showing aseries of removal processes at the point A for the liquid drops 120formed on the solid surface 110, in accordance with the presentinvention. The photographs are taken by using a commercially availablehigh-speed camera. The resonant frequency applied to the liquid drops120 is 25 Hz, approximately, wherein the vibration is induced by thetime-periodic acoustic pressure waves in accordance with the presentinvention. As shown in FIG. 3, a surface area where the liquid drops 120contact with the solid surface 110 is repeatedly increased and decreaseddepending on the vibration of the liquid drops 120 between timeintervals of −205 to −5 millisecond (ms). At time intervals of −3 to 0ms, the surface area is abruptly decreased, thereby removing the liquiddrops 120 from the solid surface 110. The time is represented as arelative time by setting a time, at which the liquid drops 120 disengagefrom the solid surface, to zero.

[0026] As described above, the apparatus in accordance with the presentinvention dramatically decreases the energy consumption of a systememploying a condensation and/or boiling process of a fluid, by enhancingthe condensation and/or boiling of the fluid in the aforementionedmanner. Also, the apparatus may be used in a radiator or cooling towerto remove the liquid drops and/or bubbles that cause erosion and scalingin the heat exchange machines. Further, the apparatus in accordance withthe present invention could be integrated as one unit so that it may beapplicable for a small-scale system employing the condensation and/orboiling process of the fluid.

[0027] While the present invention has been described and illustratedwith respect to a preferred embodiment of the invention, it will beapparent to those skilled in the art that variations and modificationsare possible without deviating from the broad principles and teachingsof the present invention which should be limited solely by the scope ofthe claims appended hereto.

What is claimed is:
 1. An apparatus for enhancing condensation andboiling of a fluid in heat exchange machines by separating at least oneof liquid drops and bubbles from a solid surface with which the fluidcontacts, the apparatus comprising: means for generating a drivingsignal based on a resonant oscillation frequency of said at least one ofthe liquid drops and the bubbles; and means, in response to the drivingsignal, for providing an acoustic pressure wave to said at least one ofthe liquid drops and the bubbles, to thereby detach them from said solidsurface.
 2. The apparatus according to claim 1, wherein the resonantoscillation frequency corresponds to a first natural oscillationfrequency ƒ₁ of each of the liquid drops, and the first naturaloscillation frequency ƒ₁ is calculated as follow:$f_{1} = {\frac{1}{2\pi}\lbrack {{n( {n - 1} )}( {n + 2} )\frac{\sigma}{\rho \quad a^{3}}} \rbrack}^{\frac{1}{2}}$

wherein n is a vibration mode number for determining the shape of avibration; and σ, ρ, and a represent the surface tension, the density,and the diameter of said each of the liquid drops, respectively.
 3. Theapparatus according to claim 1, wherein the resonant oscillationfrequency corresponds to a second natural oscillation frequency ƒ₂ ofeach of the bubbles, and the second natural oscillation frequency ƒ₂ iscalculated as follow:$f_{2} = {\frac{1}{2\pi}\lbrack {( {n + 1} )( {n - 1} )( {n + 2} )\frac{\sigma}{\rho \quad a^{3}}} \rbrack}^{\frac{1}{2}}$

wherein n is a vibration mode number for determining the shape of avibration; and σ, ρ denote the surface tension and the density of asurrounding liquid, respectively, and a is the diameter of said each ofthe bubbles.
 4. The apparatus according to claim 1, further comprisingmeans for amplifying the driving signal to a predetermined signal level.5. The apparatus according to claim 4, wherein the signal generationmeans, the providing means, and the amplification means are integratedas one unit.
 6. The apparatus according to claim 1, wherein the drivingsignal includes one of a sinusoidal, triangular, saw-tooth, andrectangular wave signals.
 7. The apparatus according to claim 1, whereinthe providing means includes a membrane, piston, acoustic speaker, andpiezoelectric device.
 8. The apparatus according to claim 2, wherein theacoustic pressure wave has a resonant oscillation frequency that dependson the first natural oscillation frequency ƒ₁, wherein the liquid dropsare vibrated with said resonant oscillation frequency.
 9. The apparatusaccording to claim 3, wherein the acoustic pressure wave has a resonantoscillation frequency that depends on the second natural oscillationfrequency ƒ₂, wherein the bubbles are vibrated with said resonantoscillation frequency.
 10. The apparatus according to claim 1, whereinthe resonant oscillation frequency is several tens of hertz (Hz).